added UserDefaultAvatar to IUser (#973)

* added UserDefaultAvatar to IUser

* pass ushort as discriminator

* removed unneeded ushort.parse

src/Discord.Net.Rest/Entities/Users/RestUser.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Diagnostics;
 using System.Threading.Tasks;
 using Model = Discord.API.User;
@@ -60,6 +60,9 @@ namespace Discord.Rest
         public string GetAvatarUrl(ImageFormat format = ImageFormat.Auto, ushort size = 128)
             => CDN.GetUserAvatarUrl(Id, AvatarId, size, format);
 
+        public string GetDefaultAvatarUrl()
+            => CDN.GetDefaultUserAvatarUrl(DiscriminatorValue);
+
         public override string ToString() => $"{Username}#{Discriminator}";
         private string DebuggerDisplay => $"{Username}#{Discriminator} ({Id}{(IsBot ? ", Bot" : "")})";